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Abstract

We describe a new four-wave rectification method for the generation of intense, ultrafast terahertz (THz) pulses from gases. The fundamental and second-harmonic output of an amplified Ti:sapphire laser is focused to a peak intensity of ∼5×1014W/cm2. Under these conditions, peak THz fields estimated at 2 kV/cm have been observed; the measured power spectrum peaks near 2 THz. Phase-dependent measurements show that this is a coherent process and is sensitive to the relative phases of the fundamental and second-harmonic pulses. Comparable THz signals have been observed from nitrogen and argon as well as from air.

Figures (3)

(a) Time-dependent THz field from FWR in nitrogen gas, (b) the associated power spectrum. The linear response of the ZnTe detector is neglected in the estimation of the field and the calculation of the spectrum. The spectrum at frequencies above 3 THz is perturbed by the 5.3-THz transverse optical phonon resonance in the ZnTe crystal used for the EO detection. Dashed curve, the spectrum anticipated from a rectified 240-fs pulse, assuming ideal detection.

Results of the phase-dependent experiments. The angle of incidence on a microscope coverslip between the BBO and the focus was tuned, and the angles associated with maxima (relative signal 1), minima
(relative signal -1), and zero crossings (relative signal 0) were recorded. Solid curve, a least-squares fit to the data.

Results of the power-dependent measurements. Solid curve, a least-squares fit to the data. A type II BBO crystal was used to generate the 400-nm pulses for this experiment. A type I crystal was used for the other measurements presented here.